In the field of electrophotographic image forming techniques, high quality image formation enabling reproduction of micro-dot images or highly precise images are demanded with the advance of digital technologies. In response thereto, there have been studied minute-particle toners. There has been also noted a so-called polymerization toner which is feasible to control particle form or size in the stage of production.
A polymerization toner is constituted of toner particles obtained by coagulating toner constituent particles such as a particulate resin obtained by a process of polymerization (e.g., emulsion polymerization), colorant particles and optionally other particles.
As one of such particulate resins constituting a polymerization toner is cited particulate styrene-acryl resin. This particulate resin is formed by an emulsion polymerization process in which a polymerizable monomer as a raw material is dispersed in an aqueous medium containing an emulsifying agent to form oil droplets, after which emulsion polymerization is undergone by adding a polymerization initiator to perform radical polymerization within the droplets, as set forth in, for example JP-A Nos. 200-214629 and 2001-125313 (hereinafter, the term JP-A refers to Japanese Patent Application publication).
A styrene-acryl resin, which exhibits a relatively low softening point due to its non-crystalline structure, is suitably used for a toner which is desired to be fixed at a relatively low temperature, a so-called low-temperature fixable toner.
However, a toner using a styrene-acryl resin exhibits superior low temperature fixability but is inferior in fixing strength, having problems that when a force such as bending or abrasion is applied onto a fixed toner image on a transfer material, this influence renders it difficult to maintain a stable toner image. In response thereto, there has been attempted stabilization by curing a resin but sufficient effects have not been achieved.
There is also cited a toner using a polyester resin. Such a toner using a polyester resin has a higher softening point than a toner using a vinyl resin but exhibits sharp melting behavior and stable fixing strength even when creased or bent so that solid toner images are advantageously obtained.
In view of the foregoing, there has been studied development of a toner having advantages of both of a styrene-acryl resin and a polyester resin. For instance, there was studied a technique for preparing a toner by incorporating a styrene-acryl resin and a polyester resin through kneading and grinding. This technique attempted to prepare a toner having a hybrid structure composed of styrene-acryl resin and polyester resin by a process of mixing both resins, followed by melting, kneading and grinding, as set forth, for example, in JP-A 2001-125313.
Recently, reduction of energy consumption in image forming apparatuses is desired from the viewpoint of global environmental conservation and enhancement of fixing efficiency in a fixing device has been studied in addition to development of a toner corresponding to low-temperature fixing. As such a fixing device, there appeared an image forming apparatus provided with a fixing device composed of a roll-form heating body and a belt-form pressure body in place of the conventional combination of a heating roller and a pressure roller, as described in JP-A 10-228198.